Vertical cavity semiconductor lasers have attracted increasing interest for a large number of applications. For example, single-mode devices with limited output power and high spectral purity are employed for sensing, pumping, spectroscopic applications and/or high-speed data communication. It should be noted that throughout this application, the term “single mode” refers to a single transverse radiation mode. Multi-mode devices, on the other hand, are used in applications demanding less spectral qualities but a higher degree of output power. A suitable measure to increase the output power of a vertical cavity surface emitting laser device resides in the possibility to manufacture a plurality of single elements on a common substrate and to arrange them as an array. Contrary to edge-emitting laser arrays, the unique configuration of vertical cavity surface emitting lasers allows the realization of densely packed two-dimensional arrays. In applications merely requiring high output power, coherence of the radiation emitted by the laser array may not play an important role. In many applications, however, low beam divergence and high spectral purity are critical parameters that do not permit the employment of non-coherent arrays. For these spectrally more demanding applications, arrays of phase-locked lasers may be arranged in such a way that the arrays select a single transverse radiation mode for the whole array. With this technique, focused diffraction-limited far-field beams can be achieved in spectrally pure, high-power single-mode VCSEL arrays.
Phase-locked VCSEL arrays have been investigated and several different designs, mostly for bottom emitting devices, have been proposed and tested. In “Continuous Wave Operation of Phase-Coupled Vertical Cavity Surface Emitting Laser Arrays” in Applied Physics Letters, volume 77, number 15, Oct. 9, 2000 by Monti di Sopra, et al, arrays of vertical cavity surface emitting lasers are described. These laser arrays are realized by patterning the reflectivity of the top distributed Bragg reflector using a phase-matching layer and a metal grid. For an improved current injection and better heat dissipation, the devices were selectively oxidized. With these arrays, a continuous wave operation at room temperature has been achieved at 960 nm.
Commonly, VCSEL devices are manufactured on a substrate having a crystallographic surface orientation [100]. When injecting current into the active region of the VCSEL, laser radiation is generated and coupled out of the device in the [100] direction. The emitted radiation typically shows a linear polarization along the [011] or [01-1] direction, except for a small elliptical component. Unfortunately, VCSELs operated in a single mode, i.e. VCSELs having an optically confining resonator that substantially favors a single transverse mode oscillation over a certain current range, tend to abruptly change the polarization direction, particularly when the injection current is increased or when the temperature is changed. These spontaneous changes of the polarization direction, i.e. from the [011] to the [01-1] direction and vice-versa, are also referred to as polarization flips. The frequency separation between the two orthogonally polarized components of the fundamental mode is between 0 and 50 GHz, depending on the device. Apart from this wavelength shift, a polarization flip also drastically increases the intensity noise. Since many applications, for example, in optical communication systems, in sensing, spectroscopy or pumping, and the like, are polarization, noise or wavelength sensitive, uncontrolled polarization flips may drastically degrade the performance of VCSEL devices, or even completely inhibit the employment of such devices for these corresponding applications.
In view of the above-mentioned problems, there exists a need for a method of operating a VCSEL device in such a way that the radiation emitted from this device is substantially coherent and spontaneous polarization flips are substantially avoided within an injection current range defining a single mode operation area.
Furthermore, a need exists for a VCSEL device exhibiting a high output power thereby substantially avoiding spontaneous polarization flips when operated as a single mode device.